土壤矿物表面催化Fe(II)氧化的动力学过程与成矿产物特性

Fe(II) oxidation and its resultant precipitation of Fe(III) minerals, is a key process determining the fate of organic matter, contaminants and nutrients in soils. In soils and sediments, Fe(II) oxidation often occurs in the presence of mineral surfaces, which are well known to catalyze the oxidation of Fe(II) and result in the formation of Fe(III) minerals with varying properties. Phyllosilicates and Fe(III) (oxyhydr)oxides are the major mineralogical components in soils. In addition, Al substitution in Fe(III) (oxyhydr)oxides is a very common process in soils. However, it is unclear how phyllosilicates and Al-substituted Fe(III) (oxyhydr)oxides affect Fe(II) oxidation kinetics and the properties of the resultant Fe(III) minerals. Accordingly, this project will investigate Fe(II) oxidation kinetics and the resulting Fe(III) solids in the presence of phyllosilicates (kaolinite, montmorillonite, illite, vermiculite and chlorite) as well as Al-substituted goethite and hematite. We will track Fe dynamics by adding 57Fe(II) to phyllosilicates and 56Fe-labeled Al-goethite and hematite, and characterize the resulting solids using 57Fe Mössbauer spectroscopy, XRD, SEM/TEM, etc. In addition, the availability of the recently precipitated Fe(III) phases toward microbial reduction will be evaluated. The objectives of this project are to (1) unravel the key mineralogical properties controlling the rates of Fe(II) oxidation, (2) assess the mineralogical nature of the newly precipitated Fe(III) minerals, and (3) elucidate the relationship between the mineralogical properties and the biological reactivity of the newly formed Fe(III) minerals. These results are essential to (1) understand the formation and evolution of Fe minerals and (2) decipher the role of Fe cycling in controlling the fate of nutrients and contaminants in natural soils.

Fe(II)氧化形成Fe(III)矿物沉淀的过程对土壤中有机质、污染物和营养元素的迁移、转化和归趋起着重要的调控作用。矿物表面具有催化Fe(II)氧化的功能，并诱导形成不同形态的Fe(III)矿物沉淀。层状硅酸盐矿物和氧化铁是土壤的主要矿物成分。氧化铁矿物中的Al同晶替代是土壤中普遍的矿物学现象。然而，层状硅酸盐矿物和Al替代氧化铁如何影响Fe(II)氧化的动力学及成矿产物性质还不明确。本项目拟通过稳定性铁同位素示踪，结合穆斯堡尔，X射线衍射、电镜等方法，研究Fe(II)在层状硅酸盐矿物、Al替代针铁矿和赤铁矿表面的非生物氧化成矿过程，明确影响Fe(II)氧化动力学的关键矿物属性，从微观结构上揭示所形成的Fe(III)沉淀的矿物组成、结晶形态、形貌等特征，阐明Fe(III)沉淀的矿物学特征与其生物稳定性的联系，为丰富环境矿物学理论、阐明铁循环对生源要素和污染物的环境行为的影响提供理论依据。

Fe(II)氧化形成次生氧化铁矿物是土壤铁循环的重要组成，这一过程也对土壤有机碳的保存及污染物的迁移转化产生重要影响。虽然已有少量实验证据证明矿物可催化Fe(II)氧化，但是土壤矿物属性及其结合态有机质如何影响Fe(II)氧化的动力学过程与成矿产物特性尚未明确。本项目采用57Fe标记及穆斯堡尔谱等手段，围绕Fe(II)aq、氧化铁和层状硅酸盐矿物体系开展研究：1.阐明了厌氧条件下游离态Fe(II)与矿物结构Fe(III)之间的Fe电子传递与原子交换机制。证实了土壤中Fe电子传递与原子交换主要发生在Fe(II)与氧化铁(而不是层状硅酸盐矿物)之间,明确了土壤有机质降低了Fe电子传递与原子交换速率，发现Fe电子传递与原子交换并未造成天然土壤氧化铁矿物的晶相转变。2.揭示了好氧条件下影响Fe(II)氧化动力学的关键矿物属性。明确了蒙脱石通过层间吸附降低Fe(II)氧化速率，而绿泥石和赤铁矿可介导电子传递过程进而催化Fe(II)氧化，并且Al同晶替代进一步提高了赤铁矿的催化功能。在此基础上，扩展到了一系列自然土壤体系中，发现土壤氧化铁矿物(水铁矿、针铁矿和赤铁矿)在催化Fe(II)氧化过程中起着主导作用。3.厘清了不同类型矿物及其结合态有机质对Fe(II)氧化成矿产物性质的调控机制。证实了层状硅酸盐矿物降低Fe(II)氧化所形成的二次矿物结晶度，从而提高了二次矿物的反应活性。发现土壤氧化铁对Fe(II)氧化形成二次矿物具有“模板效应”：针铁矿催化Fe(II)氧化形成针铁矿、赤铁矿催化Fe(II)氧化形成赤铁矿，明确了有机质有利于弱晶质氧化铁矿物形成。本项目的研究成果从机理上阐明了土壤矿物对Fe(II)氧化的制约，完善了铁循环理论体系，同时为预测和调控土壤有机碳和污染物的环境行为提供科学依据。